The manufacture of board of the type mentioned above is carried out by disintegration of the material, gluing, drying and forming into a mat, which is then hot pressed into finished board.
During the manufacture of board material, gaseous organic impurities, such as formaldehyde and other volatile hydrocarbons (VOC, or volatile organic compounds) are generated and emitted. These impurities are primarily emitted from the drying step, but to a certain extent also from the hot pressing step. These substances contribute to the greenhouse effect, and there is an increasingly stronger demand for reducing these emissions. In addition, particulate impurities, in the form of wood dust, are also emitted from the drying, and fly ash is emitted from the heat energy plant.
During the manufacture of board relatively large amounts of heat energy are consumed. The greatest heat consuming steps during the manufacture of MDF (medium density fiberboard) are the defibering process, the drying of the fiber material, and the hot pressing of the fiber mat. In the defibering process steam is used as the heating medium. The fiber dryer and press can be heated with steam, hot oil or hot water.
The fiber dryer can also be heated directly with hot flue gases from a burner in the dryer or from a common heat energy plant. The exhaust air from the drying step also contains fly ash from the flue gases.
With the use of continuous press technology the main portion of the organic substances emitted during the pressing operation can be sucked off with a limited volume of air. The exhaust air can be used as combustion air in the heat energy plant, where the organic impurities are transformed to water vapor and carbon dioxide.
Exhaust air from a dryer, however, has a much larger volume than is the volume of air from a press exhaust by suction. This fact implies that the exhaust air from the dryer cannot simply be used as combustion air in the same way as is the case with the exhaust air from the press. In the case of stringent environmental requirements it can, therefore, be necessary to apply expensive and complicated cleaning technology, for example in the form of gas scrubbers, wet electrostatic precipitators (WESP) or the use of regenerative thermal oxidation (RTO), depending on the requirements of the authorities concerned. These technologies are expensive in terms of both investment and operation costs.